A marked and permanent increase in the water exchange of the body, which we believe represents true diabetes insipidus, is caused in cats and monkeys by lesions in the hypothalamus which interrupt bilaterally the supra-optico-hypophyseal tracts. The autopsy picture in these animals has shown degeneration of these tracts in the pituitary stalk and a disappearance of the plexus formed by their fibers in the pars nervosa of the hypophysis. Atrophy of the supra-optic nuclei and of the posterior lobe is always associated with degeneration of the supraoptico-hypophyseal tract (Fisher, Ingram and Ranson;1 Fisher, Ingram, Hare and Ranson;2 Ingram, Fisher and Ranson3).
While the cats with experimental diabetes insipidus were under observation, experiments were conducted to determine the relation of the thyroid gland to the polyuria by removing the gland in three cats and by feeding thyroid to a considerably larger number. The effect on the water exchange